Prospecting for oil



Nov. 26, 1940. E. B. PECK PROSPECTING FOR OIL Filed Sept. 2, 1937 2 Sheets-Sheet 1 5 4 //1 4 r4 5 ELL: 1 u

Nov. 26, 1940. E. s. PECK v PROSPECTING ,FOR OIL Filed Sept. 2, 1937 2 Sheets-Sheet- 2 5 mu v 93 l 2 Averaye No. 02 Carbon afoms //1 HyO'IOCGI'bG S a INVENTORl PatentedNov. 26, 1-940 Edward B. Peck, Elizabeth, N. -.I., assignor to Standard Oil Development Company, a corporation of Delaware Application September 2, 1937, Serial No. 162,076

. (Cl. 23-232) The present inventlon'residesin a method for' 3 Claims.

locating sub-surface oil deposits. More particularly, it is an improvement in a known method for oil prospecting by soil gas analysis.

It hasbeen ascertained that the presence of methane in soil gas is not, per se, evidence of the existence of a sub-surface oil deposit. Such deposits are indicated by the presence in soil gas of hydrocarbons higher than methane, such as ethane, propane, etc. higher hydrocarbons in the soil gas, however, has presented a vexatious problem, since there is no chemical test which will directly distinguish ethane from methane, and the dilution these gases in the air is so extensive that isolation of the hydrocarbon constituents by liquefaction and fractionation is impossible.

The detection of these The method most commonly employed for the analysis 01' gasesior their hydrocarbon content 20 is the method known as the carbon dioxide shrinkage method. According to this method, the volume of the gas sample is first measured. It is then subjected to combustion. The amount of CO: produced is measured and the resultin 25 volume of gas is measured. The ratio of the volume shrinkage to the C0: produced is relied upon to indicate the character 01 the hydrocarbon in the gas. This method is not applicable to the problem of soil gas analysis, however, be-

cause the volume of gas required to produce a measurable jamount 01002 is so great that the volume shrinkage would be within the limits of experimental error.

According to the present a sample of soilgas in the suspected area and invention, sub-sur 8!! face deposits 0! oil aredetected by collecting subjecting this gas to atreatment suitable for establishing the concentration or and the average number of carbon atoms permolecule of 40 the hydrocarbons contained in the gas. ''More specifically, the sample oi soil gas is subjected to combustion,.- the temperature rise due to combustion or the totalheat'of combustion is measured, the carbon dioxide resulting from thecom- 4s bustion is measured so as to establish thev total carbon content of the sample "01' gas, and this carbon content is correlated with the tempera'-:

ture rise orthe total heat ct combustion, as the case-may be, to give a relation indicative so of the number of carbon atoms per. molecule of hydrocarbon in the sample.

In carrying out th .method of" the present.

'nvention, the total heat of combustion of the;

hydrocarbons in the gas sample may be meas' to ured 'by any conventional .micro calorlmeter.

- absorption of the carbon sistance is recorded on a galvanometer which is may be so calibrated as to directly record heat of combustion. Likewise, the measurement of the carbon dioxide in the products of combustion may be made in any known manner, such as by dioxide in soda lime or 10 by titration of a solution of caustic soda into which thecombustion products are fed. In order to eliminate any calculation, it is possible to employ an'amount of caustic soda just sufiicient to neutralize a molecular volume of carbon dioxide, and to run the analysis until the caustic soda is exactly neutralized.

Inasmuch as one step oithis method is an accurate measurement of the amount of carbon dioxide produced by the combustion of the hydrocarbons in the gas undergoing treatment, it is necessary that this gas beireed from any naturally occurring carbon dioxide before being burned. This is done in the conventional. manner by feeding the gas through soda limeor any other agent capable of removing'carbon dioxide. Generally, the same agent will remove anyHaS' present in the gas.

Since every apparatus will have its own peculiarities with regard to the combustion rate, 80

heat loss, etc., it is desirable, in the practice of the method of the present invention, to first calibrate the apparatus. This is done by determining total heats of combustion of samples. containing measured amounts of certain hydro- '35 carbons. For example, a gas mixture containing 600 parts per million 01' methane is, passed through the apparatus and its totalheat or combustion measured. Next, a sample containing .300 parts of ethane per million is subjected to r each' having as its'sole hydrocarbon constituent m f a difl'erent member of the methane series.

This procedure may be repeated with having diflerent carbon contents. The greater number of curves so producedthe easier will be the evaluation of the unknown sample. when the unknown sample is tested. its total heat of combustion and the amount of CO: produced by combustion are measured. The amount or carbon dioxide formed is employed in selecting the calibration curve upon which the measured heat of combustion is to be read.

49 section, oi an apparatus suitable for conducting.

The method may be somewhat simplified by so calibrating the apparatus that only the temperature rise in the combustion chamber and volumeof carbon dioxide in the products of combustion need be determined. That is to say,

in preparing the calibration chart, temperature rise in the combustion chamber for a given rate of flow is plotted against carbon dioxide content or the combustion products. For example, 1000 cu. ft. of'a gas containing a known amount of methane is passed through the apparatus at agiven velocity and the content of carbon dioxide 'in the'combustion products is measured. The 20 unknown gas, the-same volume is fed through the apparatus at the same rate. Temperature rise is measured and the amount or carbon dioxide in the combustion product is determined.

This amount of carbon dioxide is employed in selectingthe, curve on which the temperature rise is read and from which the average number of carbon atoms per molecule in the hydrocarbons contained in the gas is determined.

The present invention may be better understood by reference to the accompanying drawings in which Fig 1 is a front elevation, partly in the necessary analysis, Fig. 2 is a calibration chart showing the relation between total heat of combustion and number of carbon atoms in hydrocarbons contained in gases having the same 45 carbon content, present as hydrocarbons oi the showing the relation between temperature rise methane series, and Fig. 3 is a calibration chart in the combustion chamber with gas flowing at a specified rate and number or carbon atoms in .50 the hydrocarbons contained in the gas.

Referring to Fig. 1 in detail, a sample or gas to be tested is fed into tower I packed with soda lime or any other'reagent for the absorption of carbon dioxide.

55 through a tube 2 to combustion chamber 3. Ar-

ranged in line 2 is a .pressure control device I which controls pressure in line 2 simply by liquid level head and a flow meter 5 of conventional design.

go Arranged in combustion chamber 3 ls'anopen end 01' the tube 6 which has its inlet near the top of chamber 3 and its outlet immersed in a solution 1. ofa reagent capable of reacting with Arranged in tube 8 is a wire 8 having a thermojunction 9 locatednear the top oi tube 6 and another thermoiunction l0 located near the bottom oi. tube 0. Wires. A, B and C are attached to these junctions, wire B being common to both 70 junctions. These wires are connected ina .cir-

cult containing measuring instruments which may be calibrated to record temperature directly.

It is to'be understood that the temperature measuring arrangement described above is only .75 illustrative. Any conventional thermocouple or The purified gas then passes pyrometer may be employed for the temperature measurement so long as one measurement is made at the top or tube 6 and another at the bottom.

Of course, the instrument employed must be highly sensitive, preferably capable of recording temperature changes of at least 001C. A check may be provided on these temperature measurements by taking measurements at intermediate points.

Tube 6 is packed with pieces I l of oxidation catalyst. This catalyst may be copper oxide, a chromate, a manganite, such as manganese manganite or hopcalite. or any other of a number of known oxidation catalysts. It' may be desirable to'coat the wire used for temperature measurement with the oxidation catalyst. Combustion chamber 3 is surrounded by an electric iurnace i2, the purpose of which is to bring the combustion chamber to a temperature necessary to initiate combustion of the hydrocarbons contained in the gas sample and to maintain a substantially uniform temperature around tube 6.

The temperature diiferential between the thermocouples-in tube 6 gradually reach a steady statewith constant flow of gas and it is this measurement. It is not always possible to reach a steady state in tube 8 with a given volume or gas flowing through the apparatus, therefore through stop cock is the flow oi gas-may be directed to either absorber vessel l3 or 83' until a steady state is reached, after which the stream may be diverted through vessel l3 where the total volume of gas required to produce a deflnite amount of CO2 may be measured.

The vessel l3 carrying the solution for measuring the CO: may be replaced by a tube packed with a solid material, such as soda lime, capable oi absorbing CO2. When this expedient is employed, it may be advisable to interpose between the combustion chamber and the CO2 absorption tube a tube packed with a dehydrating agentv such as manganese perchlorate. It is advisable to provide the exhaust tube II. which conducts the remainder of the combustion product from the system, with a water seal of conventional design or any other device for preventing the leakage of atmospheric gases into the apparatus from that end.

In Fig. 2 is shown a series of curves obtained by plotting total heat. 01' combustion per unit volume of gas against the number of carbon atoms in the hydrocarbons in the gas. Each curve represents these values for a given percentage oi carbon dioxide in the combustion gas. These curves are obtained in the manner previously described. In utilizing these curves for the determination or the average number of carbon atoms per molecule of hydrocarbon in a sample of gas, a volume oi gas corresponding to the volume employed for the production or the curves is subjected to combustion. The total heat of combustion per unit volume of this gas'ls determined and the amount of CO2 in the combustion gas per unit volume of feed gas is measured. The amount of CO2 round is utilized to select the curve upon which the determined heat 01' combustion is read. 7

Let it be assumed that the curves were produced by burning the necessary number of 10 liter batches of gases of known composition. Let it be further assumed that 10 liters of sample gas evolved 30 gram-calories of heat per liter and produced 20 ccs of 00: per liter. The total heat of combustion, when read 'on curve "B,"

final steady temperature which characterizes the 1.6. In other words, the sample gas would contaln hydrocarbons higher than methane.

If there is no curve corresponding to the measured quantity of CO2, a curve can be interpolated between and substantially parallel to the two curves representing the next higher and next lower CO2 content.

In Fig. 3 is shown a series of curves obtained by plotting temperature rise at a given rate of flow against number of carbon atoms per molecule in the hydrocarbon contained in the gas undergoing test. Here again each curve represents the relationship between these values for a given volume of CO2 in the exhaust gas. In this case, as previously pointed out, sample gas must be fed through the apparatus at the same rate as that employed with the gases of known composition used as the basis for the curves.

Also contemplated within the scope of the present invention is a method in which it is not necessary to measure the carbon dioxide produced from each sample of soil gas. Experiment has shown that over a given area, unless the area includes a swamp, the methane content of the soil gas remains substantially constant. In such areas it is possible, according to the present invention, to detect changes in the content of the soil gas in parafiin hydrocarbons heavier than methane by simply passing samples of soil gas obtained at different points in the area through a combustion chamber at a selected rate of flow and measuring the rise in temperature due to combustion. In this procedure, those points at which marked differences in temperature rise occur can be selected for a more a'ccurate determination of the nature of the hydrocarbons in the samples collected at these points. It is, of course, obvious, that, instead of passing the samples of gas from the different points through a. combustion chamber at the same rate of speed, the same result can be obtained by subjecting the same volume of gas from the respective points to combustion and determining the heat of combustion of the several samples.

The present invention is best employed in conjunction with known geological or geophysical methods for identifying sub-structures favorable to the accumulation or oil. For example, when a salt dome has been located by any one 01 the conventional geophysical methods,- a sample of soil gas is taken at a depth of about feet, directly over the peak of the dome, and other samples are taken at the same depth at points spaced from the first sampling point. The point at which a sample having the highest average number or carbon atoms per molecule of hydrocarbon is round is then selected for the drilling operation.

worinng according to the present invention, the average number of carbon atoms per hydrocarbon molecule in the soil gas becomes significant when it exceeds about 1.2. Generally an oil deposit will be indicated by an average number of carbon atoms per hydrocarbon molecule in soil gas ranging from about 1.5 to about 2.2. An average number of carbon atoms per hydrocarbon molecule in soil gas less than 1.2 is generally indicative of a younger formation such as a swamp or peat bed.

Having thus described the nature and objects of the present invention, what is claimed as new and us'efuland as desired to secure by Letters posits which comprises collecting samples of soil gas at difierent points in an area under investigation, passing each sample through a combustion chamber maintained under conditions suitable ior the combustion of the hydrocarbon content of the sample, measuring the temperature rise in the combustion chamber due to the combustion of the sample, comparing the combustion effects of the several samples to thereby determine the points at which samples having the greatest combustion effects were obtained, and subjecting gases from these points to examination for hydrocarbons higher than methane.

2. A method for locating sub-surface oil deposits which comprises collecting samples of soil gas at a number of points in an area under investigation, passing each sample at the same selected rate of flow through a combustion chamber maintained under conditions suitable for the combustion of the hydrocarbon content of the sample, comparingthe increases in temperature in the combustion chamber due to the combustion of the several samples to thereby determine the points at which samples having the greatest combustion effects were obtained and determining the carbon-hydrogen ratio of gases collected.

gas at different points in an area under investigation, passing each sample, without pretreatment,-at the same selected rate of flow through v a combustion chamber maintained under conditions suitable for the combustion of the hydrocarbon content of the sample, measuring the temperature rise in the combustion chamber due to the combustion of the sample, comparing the I increases in temperature in the combustion chamber due to the combustion of the several samples to thereby determine the points at which samples having the greatest combustion efl'ects were obtained and determining the carbon-hydrogen ratio or gases collected at these mints.

EDWARD B. PECK. 

